Google co-founder Sergey Brin admits he had been proven wrong in questioning China's ability to restrict the free flow of information online, and is "most concerned" by the Internet censorship efforts of repressive states such as China, Saudi Arabia, and Iran.

The Guardian reported on Monday that the Google executive said he did not believe a country such as China could effectively restrict Internet freedom for long five years ago. To that, he has been proven wrong and is "more worried than [he] has been in the past".

"I thought there was no way to put the genie back in the bottle, but now it seems in certain areas the genie has been put back in the bottle," he said in the report.

Even though Google still has a presence in China and has been adding to its team of engineers in the country, it remained critical of the government's hardline stance on Web censorship since its high-profile decision to relocate its search business to Hong Kong in early 2010, Brin remarked.

He also stated he was "most concerned" by the efforts of countries, including China, Saudi Arabia and Iran, to censor and restrict the use of the Internet. There are "very powerful forces that have lined up against the open Internet on all sides and around the world", he warned.

The threat to online freedom comes from a combination of factors, which include governments increasingly trying to control their citizens' access and communications, the entertainment industry's attempts to crack down on privacy, and the rise of "restrictive" companies such as Facebook and Apple that strictly controls what software are made available on their platforms, he noted.